Who is regarded as a citizen and the rights of such citizenship are determined by each country, which sets its own criteria for citizenship. A person concurrently regarded as a citizen of more than one state under the laws of these states is one who has multiple citizenship, also known as dual citizenship or multiple nationality.
National laws may include criteria as to specific circumstances in which someone may concurrently hold another citizenship. A country may withdraw its own citizenship, if a person acquires the citizenship of another country.
The nationality (citizenship) of a person isn’t governed by international convention but rather by national laws, which vary and can be inconsistent with each other. People are known to “hold” multiple citizenship, but in fact each nation makes a claim that a specific person be regarded as its national.
Some countries do not permit dual citizenship. Most countries that permit dual citizenship may not recognize the other citizenship of its nationals within its own territory in relation, for instance, to national military service, duty to vote etc. (Some countries do not allow dual citizenship holders from serving in their military, police or specific public service positions.) Countries may not allow consular access by another country for someone who is also its national. (Pikamalt Eesti Elu 27. juuni paberlehes)
Citizenship, a notion undergoing adjustment in many different countries (III)